Immune-mediated thrombocytopenias are the leading cause of hemorrhagic diathesis with a prevalence >0.1%. Autoimmune idiopathic thrombocytopenic purpura (ITP) is the most common, but sensitization to alloantigens and drugs cause some of the severest thrombocytopenias. Mechanisms underlying these disorders are obscure. We found ITP antibodies (Abs) to be directed against platelet membrane glycoproteins (GPs) IIb/IIIa and Ib/IX equally and less often against GPs Ia/IIa and IV; IgG and IgA classes were present equally and IgM rarely. Abs against exosolic domains of GPs were unique to ITP. Abs to cytosolic domains were a nonspecific secondary phenomenon of platelet destruction. Five GP peptides of 12 to 23 amino acids, selected for potential immunogenicity, defined epitopes recognized by different ITP sera. Further epitope analysis is underway using recombinant GP fragments. Posttransfusion purpura (PTP) was found to be caused by adsorption of platelet membrane material from transfused blood to recipients' platelets which are then destroyed by alloAbs that develop against the transfused material. The nonsedimentable material was both particulate (<0.1 mum), associated with a group of platelet- and plasma-derived proteins, and soluble, in association with phospholipids. Human platelet membrane- derived material coated guinea pig platelets in vivo and as little as 200 molecules of antigen per cell resulted in platelet destruction by human alloAbs. By electron microscopy using gold-labeled Ab probes, human material absorbed by guinea pig platelets was distributed like integral GPs on human platelets. In drug hemolytic anemia, 2 cases caused by stibophen and one by tolmetin had drug-dependent antibodies that reacted with erythrocyte band 3. No major blood group substance was involved in the reaction. The submolecular component(s) participating in the drug-dependent reaction will be investigated utilizing fragmented and enzyme-altered band 3.